1. Field of the Invention
The present invention relates to DC-to-AC converters for generating an alternating current from a direct current and, in particular, relates to DC-to-AC converters having an over-current protection feature, which prevents the output current from becoming higher than an over-current set value.
2. Description of the Related Art
A known DC-to-AC converter is described in Japanese Laid-open Patent Publication No. 9-247950 and is shown in FIG. 3. This DC-to-AC converter generates an alternating current from a direct current voltage, which is supplied, e.g., by a vehicle battery. The known DC-to-AC converter has an input filtering circuit 1, a voltage boosting circuit 2 and a direct current/alternating current (DC-TO-AC) converting circuit 3.
The input filtering circuit 1 reduces noise from the input terminals TA1 and TA2 attached to the direct current voltage and reduces noise generated in the DC-to-AC converter that is outputted externally through the input terminals TA1 and TA2.
The voltage boosting circuit 2 boosts the direct current voltage conducted via the input filtering circuit 1. That is, by alternately turning on and off field-effect transistors TR1 and TR2, the first control circuit CR1 generates an alternating current in the secondary side coil of the transformer T1 and the voltage of the alternating current corresponds to the winding ratio between the primary side coil and the secondary side coil. Rectifier RE rectifies the alternating current to charge capacitor C3.
The DC-to-AC converting circuit 3 converts the voltage across capacitor C3 into an alternating current of a predetermined frequency by alternately turning on and off two sets of field-effect transistors (TR3, TR4 and TR5, TR6) and outputs the alternating current to a load.
In addition, a current detecting resistor R1 detects the output current IL from the DC-TO-AC circuit 3. The second control circuit CR2 turns off transistors TR3 and TR5, thereby disconnecting the output current IL, when the output current IL detected by the current detection resistor R1 exceeds an over-current set value OC. The second control circuit CR1 turns on the transistors TR3 and TR5 again when the output current IL drops below the over-current set value OC. In addition, transistors TR4 and TR6 can be turned off as well. Thus, by preventing the output current from exceeding the over-current set value, the transistors (TR3-TR6) are protected from damage caused by excess current.
Such a known DC-to-AC converter can be utilized to power, e.g., a television set, which usually has a power source circuit as shown in FIG. 4. This circuit includes a rectifier D for rectifying an alternating current applied to the input terminals TA5 and TA6, an electrolytic capacitor C having a large capacity to smooth the direct current voltage rectified by the rectifier D and other elements. The voltage stored on capacitor C can be supplied to the respective circuits in the television set, to voltage monitoring circuits of a microcomputer MC, and other circuits. In addition, a relay RL is typically connected between the input terminal TA6 and the rectifier D. If the microcomputer MC detects an abnormality, such as the voltage of capacitor C does not reach a predetermined voltage within a fixed period of time after the power source switch (not illustrated) of the television set has been turned on, the microcomputer MC disconnects the relay RL. Therefore, the television power source circuit will be disconnected from the AC power source. The power source circuit shown in FIG. 4 exhibits capacity load characteristics, because the capacity of the electrolytic capacity C is large.
Problem with the Related Art
When the power source circuit shown in FIG. 4 is connected to the DC-to-AC converter shown in FIG. 3, the television may not operate. For example, if the input terminals TA5 and TA6 of the power source circuit shown in FIG. 4 are connected to the output terminals TA3 and TA4 of the DC-to-AC converter shown in FIG. 3 and the television power source switch is turned on, an alternating current voltage will be supplied from the DC-to-AC converter to the power source circuit of the television. If capacity C is in a state of discharge, as is common when the television has not been used for a period of time, the impedance of the capacity C is relatively small. Therefore, a large current will initially flow into capacitor C when the television power switch is turned on. However, if the initial current (i.e., output current IL) exceeds the over-current set value OC, the second control circuit CR2 will turn off transistors TR3-TR6 and thereby disconnect the output current IL from capacitor C.
As shown in FIG. 5, the output current IL is disconnected at time t2, which is slightly delayed from time t1 when the output current IL exceeded the over-current set value OC. Shortly thereafter, the output current IL will become less than the over-current set value OC and the second control circuit CR2 will again turn on transistor TR3 or TR5 (and transistor TR4 or TR6). Consequently, the output current IL will begin flowing again at time t4 (when the output current IL is nearly zero), which is slightly delayed from time t3 when the output current IL, became less than the over-current set value OC (See FIG. 5). Once the output current IL exceeds the over-current value OC, the output current will be disconnected again. This "ON/OFF" operation may continue for a substantial period of time.
When the output current from the DC-to-AC converter is disconnected, charge supplied to capacitor C shown in FIG. 4 becomes less. As a result, the time for capacitor C to reach a predetermined voltage is longer than if capacitor C is continuously charged. If the voltage of capacitor C does not reach the predetermined voltage within the predetermined time, the television will not operate, because the microcomputer MC of the television set will disconnect the television from the DC-to-AC converter by turning off the relay RL.
One possibility for overcoming this problem is to increase the over-current set value in order to charge capacitor C more quickly. However, this technique is not particularly useful, because it will become necessary to use switching elements having higher-rated current thresholds, which switching elements are generally more expensive.